Oxidative Depolymerization Of Cornstalk Lignin Via ZrCl₄/NaOH In Acetonitrile/Water

Lignin is a natural aromatic polymer,as well as the only renewable aromatic resource in nature.It can be used as an alternative to fossil resources to produce valuable aromatic platform chemicals,liquid fuels,and biomass-based materials.Compared with cellulose,lignin has the complex spatial structure linked by C-C and C-O bonds with high bond-dissociation energy,which makes it different to be converted and utilized.Among these viable methods for lignin valorization,oxidative depolymerization is one of the promising approaches that can convert lignin into value-added aromatic compounds.In the thesis,we focused on the oxidative cleavage of the most abundant?-O-4 linkages in lignin,and investigated the effects of various solvents and catalysts under mild conditions.The optimized catalytic oxidation systems were further employed for the oxidative depolymerization of corncob lignin and cornstalk.The following results were obtained.The catalytic oxidation of lignin dimeric model compound 2-phenoxy acetophenone via various catalysts in different solvent systems was first investigated.Among these catalysts,ZrCl₄showed the best catalytic ability for the aerobic oxidation of 2-phenoxy acetophenone with the recovery of phenol and benzoic acid reaching 40.2%and 93.5%respectively.Moreover,the highest yield of monomers were obtained when the volume ratio of acetonitrile to water was 1:1.After catalytic oxidation with 8 h at 140°C,92 mol%of 2-phenoxy acetophenone was converted,and the corresponding yields of phenol and benzoic acid were respectively 37 mol%and 86 mol%.Generally,in acetonitrile/water?v:v=1:1?co-solvent,the high-efficiency cleavage of?-O-4 bond in2-phenoxy acetophenone can be achieved with ZrCl₄ as the catalyst and oxygen as the oxidant.The effect of reaction time on the yields and selectivity of produced monomers,as well as the structural evolution of corncob lignin during the oxidative depolymerization catalyzed by ZrCl₄/NaOH in acetonitrile/water were further investigated.Results showed that after the oxidative depolymerization with 2 h,the total yields of monomers were 6.8 wt%,in which the selectivity of vanillin and 4-hydroxybenzaldehyde,two dominant products,reached 31.47%and 26.42%,respectively.The MALDI-TOF/MS indicated that oligomers were accompanied produced together with the monomers,and the long reaction time would further destroy some high-molecular-weight oligomer substructures into low-molecular-weight species.The variation of weight-average molecular weight with reaction time confirmed the presence of dimers and oligomers.In addition,quantitative ¹³C and 2D HSQC NMR demonstrated that the ZrCl₄/NaOH-catalyzed oxidation occurred on the inter-unit bonds,especially the?-O-4 linkages,but not on the aromatic units.The oxidative depolymerization of original cornstalk lignin catalyzed by ZrCl₄/NaOH in acetonitrile/water system was also studied.The effect of reaction time,reaction temperature,amount of ZrCl₄,amount of NaOH,acetonitrile/water ratio,and oxygen pressure were tested systematically.The optimized conditions for the oxidative depolymerization of original cornstalk lignin were as follows:1 g cornstalk,15 wt%ZrCl₄,8 wt%NaOH,30 mL acetonitrile/water?v:v=1:1?,1.5 MPa O₂,140°C,2 h.The optimal yields reached 7.6 wt%for identified monomers and 5.6 wt%for unknown monomers.In addition,the Py-GC-MS results of solid residues obtained from the optimal conditions and cornstalk showed that the lignin component within cornstalk had been almost degraded.